A breakdown of law and order in the flood-hit Haitian city of Gonaives is hampering efforts to distribute food, UN relief workers have said.
The UN has about 750 troops in Gonaives
The UN's humanitarian agency has appealed for extra forces to police Gonaives, where a hurricane has left 200,000 people starving and homeless.
Armed gangs roam the flooded streets, shooting people in their search for food and looting aid lorries.
More than 2,000 people may have been killed by Hurricane Jeanne.
More than 1,500 are known to have died in the port city while about 900 are missing, many of them possibly washed out to sea during the storm 10 days ago.
Peacekeepers, deployed in Haiti after civil unrest earlier this year, have fired into the air and used tear gas in recent days in an effort to prevent looting.
"It's very difficult to get food," local resident Manette Jean was quoted as saying by the Associated Press news agency on Tuesday.
"We come every day... People are getting very frustrated."
The 3,000-strong UN peacekeeping force in Haiti has appealed for a further 3,700 soldiers to bring it up to strength while Haiti's tiny police force - 3,000 officers for a population of 8m - has little impact.
A spokeswoman for the UN's Office for the Coordination of Humanitarian Affairs (OCHA) said that aid distribution was being hindered by the lack of security.
A city drenched with mud is scrabbling for clean water
"We need more soldiers to ensure distribution is carried out calmly," Elizabeth Byrs told journalists.
"We need more for the safety, and the secure distribution of food, for these people who have been starving for five days."
Ms Byers said in Geneva that the UN had only two food distribution points in operation whereas it needed at least six.
In Gonaives itself, the security chief for the UN stabilisation mission in Haiti, John Harrison of the Royal Canadian Mounted Police, said gunmen were active in the city's port area.
"There's a big problem with gangs," he said.
"I think things could get worse... There is no security right now. There are no patrols. There are no functioning police. We're on our own."
City in fear
An AP correspondent who visited the Cassolet slum of Gonaives found a climate of violence.
A local gang member, Rony Coq of the Bottle Army gang, said local people had to "fight for everything".
Mud five feet (1.5 metres) deep still mired streets in the slum where unclaimed dead bodies have been buried in mass graves.
AP's reporter witnessed a human skeleton protruding from a mound of sludge in the street.
A lack of clean drinking water is adding to the food crisis in Gonaives, a city of about 250,000 people, as one mother told Reuters news agency.
"We don't know if the water is good, but we have to use it - if we don't cook anything my children are going to die," said Jacqueline Orassin, a mother of six.
Martha Casseus, a 16-year-old girl, wept as she described how two men had stolen a bag of rice and two jugs of cooking oil from her.
"I waited all that time and now I am going home empty-handed," she said.
"I don't know what I am going to eat. My mother is sick. She has not eaten for days. What am I going to tell my mother?"